A sintered R-T-B based magnet, including R2T14B type compound crystal grains as main phases, is known as a permanent magnet with the highest performance, and has been used in various types of motors such as a voice coil motor (VCM) for a hard disk drive and a motor for a hybrid car and in numerous types of consumer electronic appliances.
As a sintered R-T-B based magnet loses its coercivity at high temperatures, such a magnet will cause an irreversible flux loss. For that reason, when used in a motor, for example, the magnet should maintain coercivity that is high enough even at elevated temperatures to minimize the irreversible flux loss.
It is known that if R in the R2T14B type compound crystal grains is replaced with a heavy rare-earth element RH (which may be Dy or Tb), the coercivity of a sintered R-T-B based magnet will increase. It is effective to add a lot of such a heavy rare-earth element RH to the sintered R-T-B based magnet to achieve high coercivity at a high temperature. However, if the light rare-earth element RL (which may be Nd or Pr) is replaced with the heavy rare-earth element RH as R in a sintered R-T-B based magnet, the coercivity certainly increases but the remanence decreases instead. Furthermore, as the heavy rare-earth element RH is one of rare natural resources, its use should be cut down.
For these reasons, various methods for increasing the coercivity of a sintered magnet effectively with the addition of as small an amount of the heavy rare-earth element RH as possible have recently been researched and developed in order to avoid decreasing the remanence. The applicant of the present application already disclosed, in Patent Document No. 1, a method for diffusing a heavy rare-earth element RH inside of a sintered R-T-B based magnet body while supplying the heavy rare-earth element RH onto the surface of the sintered R-T-B based magnet body (which will be referred to herein as an “evaporation diffusion process”). According to Patent Document No. 1, inside of a processing chamber made of a refractory metallic material, the sintered R-T-B based magnet body and an RH bulk body are arranged so as to face each other with a predetermined gap left between them. The processing chamber includes a member for holding multiple sintered magnet bodies and a member for holding the RH bulk body. A method that uses such an apparatus requires a series of process steps of arranging the RH bulk body in the processing chamber, introducing a holding member, putting the sintered magnet bodies on a net, mounting the holding member on the sintered magnet bodies, putting the upper RH bulk body on the net, and sealing the processing chamber hermetically and carrying out an evaporation diffusion.
Patent Document No. 2 discloses that in order to improve the magnetic properties of an R-T-B based intermetallic compound magnetic material, a powder of Yb metal with a low boiling point and a sintered R-T-B based magnet body are sealed and heated in a thermally resistant hermetic container, thereby depositing uniformly a coating of Yb metal on the surface of the sintered R-T-B based magnet body and diffusing a rare-earth element inside of the sintered R-T-B based magnet body from that coating (see, in particular, Example #5 of Patent Document No. 2).
Patent Document No. 3 discloses conducting a heat treatment process with a ferrous compound of a heavy rare-earth compound including Dy or Tb as a heavy rare-earth element attached to a sintered R-T-B based magnet body.